The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A constant velocity joint is structured such as to joint an input shaft and an output shaft which are provided so as to change an intersecting angle therebetween, in such a manner that a constant velocity rotation can be transmitted to the output shaft from the input shaft, and a housing (an outer race) accommodating a bearing part is provided in one shaft. In order to seal a lubricant (generally, a grease) inside of the joint, and prevent a water, a muddy water or the like from making an intrusion into the inside of the joint, a constant velocity joint boot is installed. The constant velocity joint boot has an elastically deformable bellows part, a large-diameter cylinder part formed in one end in an axial direction of the bellows part, and a small-diameter cylinder part formed in the other end in an axial direction of the bellows part, and is structured such that the large-diameter cylindrical part is fixed to a housing, and the small-diameter cylindrical part is fixed to the other shaft. Further, there has been developed a constant velocity joint boot made of a thermoplastic resin for the purpose of a weight saving.
As a method of manufacturing an article corresponding to an intermediate product of the boot, there has been known an injection blow molding. The injection blow molding is a method of molding a parison in accordance with an injection molding and executing a blow molding after stabilizing a shape of the parison. A description will be given of a representative example of the injection blow molding with reference to FIG. 7.
An apparatus used for the injection blow molding is provided with a rotating body 100 having a core cylinder 101 in each ends, as shown in FIG. 7. First, a first outer mold 200 for an injection molding is closed with respect to the core cylinder 101, and a thermoplastic-elastomer 501 in a fluid state is injected into a cavity formed by the core cylinder 101 and the first outer mold 200 by an injection mechanism 400 (FIG. 7A). When a temperature of the thermoplastic-elastomer within the cavity is decreased, a shape is stabilized at a certain degree, and a parison 502 is molded (FIG. 7B). Thereafter, the first outer mold 200 is released while the parison 502 has a certain degree of retention heat, and the rotating body 100 is rotated at 180 degree (FIG. 7C). Further, a blow-up is executed by closing a second outer mold 300 for blow molding in a state in which the parison 502 is attached to the core cylinder 101, and an article 503 is molded (FIG. 7D). Thereafter, the second outer mold 300 is released, and the article 503 is released from the core cylinder 101 (FIG. 7E).
The manufacturing method described here is a manufacturing method (a hot parison method) of executing the blow-up by utilizing the retention heat at a time of injection molding the parison, however, there is a manufacturing method (a cold parison method) of executing the blow-up by cooling until the shape of the parison is sufficiently stabilized and thereafter reheating the parison.
It is possible to obtain a constant velocity joint boot 504 corresponding to a final product by cutting a leading end part of the article 503 formed as mentioned above. FIGS. 8A and 8B are schematic views of the constant velocity joint boot, in which FIG. 8A is a top elevational view and FIG. 8B is a front elevational view. The constant velocity joint boot 504 is generally constituted by a bellows-shaped body part, and cylindrical parts in both ends thereof. In this case, the cylindrical parts in both ends are formed at a time of injection molding, and are fitted into the second outer mold 300 in such a manner as to be prevented from being deformed at a time of the blow molding. On the contrary, the body part is shaped at a time of the blow molding. In other words, in FIG. 8, the constant velocity joint boot 504 is structured such that a range shown by reference symbol A (the body part) is shaped in accordance with the blow molding, and a range shown by reference symbol B (the large-diameter cylindrical part) and a range shown by reference symbol C (the small-diameter cylindrical part) are shaped in accordance with the injection molding.
In this case, since an annular end surface part (an annular shoulder part between the body part and the large-diameter cylindrical part) in an axial direction of the large-diameter cylindrical part of the parison 502 seals between the core cylinder 101 and the second outer mold 300 so as to prevent a leakage of a blow air at a time of the blow-up, it is not necessary to completely contact an inner peripheral surface of the second outer mold 300 and an outer peripheral surface of the large-diameter cylindrical part so as to clamp mold, but a predetermined clearance X is provided between the inner peripheral surface of the second outer mold 300 and the outer peripheral surface of the large-diameter cylindrical part as shown in FIG. 10. However, if the clearance X is too large (FIG. 10B), a gap is generated between the core cylinder 100 and the parison 502 at a time of the blow-up (FIG. 10C), and the blow air leaks from the gap. Accordingly, a sufficient pressure is not applied to the inner surface of the parison so as to cause a blow short. Therefore, in the case of taking the blowing performance into consideration, it is necessary to design the second outer mold in such a manner that the clearance X becomes as small as possible (it is desirable to design the metal mold in such a manner that the clearance X comes to 0 in a state of mold clamping the parison). However, it is hard to design while accurately forecasting the clearance X for the reasons (1) an expansion and a deformation of the resin due to a heating in the case that a reheating step of the parison is provided, (2) a difference of a linear expansion amount of the metal mold due to a difference between an assumed temperature in a design stage and a temperature at a time of actually molding, (3) a positioning accuracy of the blow metal mold and the core cylinder, (4) an attaching accuracy of the mold and the like.
On the contrary, if the clearance X is small, a mold biting is generated at a parting line P at a time of mold clamping the second outer mold 300 as shown in FIG. 9 (a part surrounded by a middle point line in FIG. 9). If the mold biting is generated, the large-diameter cylindrical part of the article is deformed, and a grease seal performance at a time of being used as the constant velocity joint boot is lowered. Accordingly, in order to prevent the mold biting, it is necessary to set the clearance X as large as possible. However, if the clearance X is large, the blow air leaks as mentioned above, and the blowing performance is lowered. Further, it is impossible to comply with a product specification. In other words, if the clearance X is made small for increasing the blowing performance, the mold biting is generated, and if the clearance X is made large for preventing the mold biting, the blowing performance is lowered.